Existing problems and solutions of PE and PPR pipeline extrusion
Existing problems and solutions of PE and PPR pipeline extrusion1. The basic principle of plastic extrusion
The plastics processing industry is a very comprehensive technology-based industry. It involves polymer chemistry, polymer physics, interface theory, plastic machinery, plastic processing molds, formula design principles and process control. Extrusion theory mainly studies the movement and change of plastics in the extruder. The relationship between the three physical states of the polymer in the extruder under a certain external force in different temperature ranges, the screw structure, plastic properties, and processing conditions. In order to carry out reasonable process control. In order to achieve the purpose of improving the output and quality of plastic products. Plastic polymer materials, when heated under a constant pressure, appear in three physical states: glass state, highly elastic state, and viscous flow state in different temperature ranges. The molding temperature of general plastics is above the viscous flow temperature.
2. Polyolefin pipeline extrusion process control
The control parameters of the extrusion molding process include the molding temperature, the working pressure of the extruder, the screw speed, the extrusion speed and the traction speed, the feeding speed, the cooling setting, etc.
1. Raw material pretreatment
Polyolefins are non-absorbent materials, usually with low moisture content, which can meet the needs of extrusion, but when polyolefins contain water-absorbing pigments, such as carbon black, they are sensitive to humidity. In addition, when using regrind and filler, the water content will increase. Moisture not only causes the inner and outer surfaces of the pipe to be rough, but may also cause bubbles in the melt. Raw materials should usually be pretreated. Drying treatment is generally adopted, and corresponding additives with dehumidification function can also be added. Such as anti-foaming agent. The dry temperature of PE is generally 60-90 degrees. At this temperature, the output can be increased by 10%-25%.
2. temperature control
Extrusion molding temperature is a necessary condition to promote plasticization of molding materials and plastic melt flow. It has a very important influence on the plasticization of materials and the quality and output of products. The theoretical temperature window of plastic extrusion is between the viscosity flow temperature and the degradation temperature. For polyolefins, the temperature range is wide. Usually above the melting point, can be processed below 280 degrees. To correctly control the extrusion molding temperature, we must first understand the relationship between the temperature limit of the processed material and its physical properties. To find out its characteristics and rules, we can choose a better temperature range for extrusion molding. Therefore, the following aspects should be considered in the temperature setting of each section: First, the properties of the polymer itself, such as melting point, molecular weight size and distribution, melt index, etc. Second, consider the performance of the device. In some equipment, the temperature of the feed section has a great influence on the current of the host. Again, by observing whether the surface of the tube die extruded tube blank is smooth. Determine whether there are bubbles or other phenomena.
The extrusion temperature includes the set temperature of the heater and the melt temperature. The heating temperature refers to the temperature provided by the external heater. Melt temperature refers to the temperature of the material between the front section of the screw and the head.
The barrel temperature distribution, from the feeding area to the die head, may be flat distribution, increasing distribution, decreasing distribution and mixed distribution. Mainly depends on the material point and the structure of the extruder.
The temperature of the machine head is set. In order to obtain better appearance and mechanical properties, and to reduce the expansion of the melt outlet, the temperature of the body is generally controlled to be lower, and the temperature of the head is higher. The temperature of the machine head is too high, so that the material can smoothly enter the mold, but the shape stability of the extrudate is poor, and the shrinkage rate increases. If the temperature of the machine head is low, the plastic material is bad, the melt viscosity is large, and the pressure of the machine head rises. Although this will make the product too dense, the post-shrinkage rate is small, and the shape stability of the product is good, it is difficult to process, the die swell is large, and the product surface is rough. It will also increase the back pressure of the extruder, the equipment load is large, and the power consumption will also increase.
The setting temperature of the die, the temperature of the die and the core die have an effect on the surface finish of the tube. Within a certain range, the temperature of the die and the core die is high, and the surface finish of the tube is high. Generally speaking, the temperature of the die outlet should not exceed 220 degrees, the melt temperature at the inlet of the nose should be 200 degrees, and the temperature difference between the melt at the inlet and the outlet of the die should not exceed 20 degrees. Because the higher temperature difference between the melt and the metal will cause the shark skin phenomenon. Too high melt temperature leads to die build-up. But it depends on the actual situation.
Melt temperature refers to the actual melt temperature measured at the end of the screw and is therefore the dependent variable. Mainly depends on the screw speed and barrel setting temperature. The upper limit of the melt temperature of polyethylene pipe extrusion is generally specified as 230 degrees. Generally, it is better to control at about 200 degrees. The upper limit of melt temperature of polypropylene pipe extrusion is generally 240 degrees. The melt temperature should not be too high. Generally consider the degradation of the material, and at the same time, the temperature is too high will make the shaping of the pipe difficult.
3. Pressure control
The most important pressure parameter in the extrusion process is the melt pressure, that is, the head pressure. Generally speaking, increasing the melt pressure will reduce the output of the extruder and increase the density of the product, which will help to improve the quality of the product. But too much pressure will cause safety problems. Melt pressure is related to raw material performance, screw structure, screw speed, process temperature, mesh size of filter screen, porous plate and other factors. The melt pressure is usually controlled between 10-30MPa.
4. Vacuum setting
Vacuum setting mainly controls two parameters, vacuum degree and cooling rate. Generally, under the premise of satisfying the appearance quality of the pipe, the vacuum should be as low as possible, so that the internal stress of the pipe is small, and the product is deformed during storage.
5. cool down
The temperature requirements of the cooling water in the extrusion of polyethylene pipes are generally low, usually below 20 degrees. When producing PPR pipes, the temperature in the first stage can be slightly higher and the latter stage is lower, thereby forming a temperature gradient. It is also very important to adjust the cooling water flow. If the flow rate is too large, the surface of the pipe will be rough, resulting in spotted pits. If the flow rate is too small, bright spots on the surface of the pipe are easily broken, such as uneven distribution, uneven wall thickness, or ellipse.
6. Screw speed and extrusion speed
The screw speed is a heavy industry parameter that controls the extrusion rate, output, and product quality. The rotation speed of the single screw extruder increases and the output increases. As the shear rate increases, the apparent viscosity of the melt decreases. Conducive to the homogenization of materials. At the same time, due to good plasticization, the intermolecular force increases and the mechanical strength increases. However, the screw speed is too high, the motor load is too large, the melt pressure is too high, the shear rate is too high, the die swelling increases, the surface becomes bad, and the extrusion amount is unstable.
7. Traction speed
The traction speed directly affects the product wall thickness, dimensional tolerances, performance and appearance. The traction speed must be stable, and the traction speed matches the pipe extrusion speed. The ratio of the traction speed to the linear extrusion speed reflects the degree of orientation that the product may occur. The ratio is called the draw ratio, and its value must be equal to or greater than 1. The traction speed increases, and the traction speed is fast when the temperature conditions for cooling and setting remain unchanged , The product stays in the sizing sleeve and the cooling water tank for a relatively short time. After cooling and setting, there will be more heat inside the product, which will cause the orientation structure of the product that has been formed during the traction process to be deoriented , Which causes a decrease in the degree of product orientation. The faster the traction speed, the thinner the wall thickness of the pipe, and the greater the shrinkage of the cooled product in the longitudinal direction. The slower the traction speed, the thicker the pipe wall thickness, the easier it is to cause material accumulation between the die and the sizing sleeve. Destroy normal extrusion production. Therefore, the extrusion speed and traction speed in extrusion molding must be well controlled.
8. Online quality control and post-processing of pipes
Polyolefin is a crystalline polymer, and the performance of the pipe just off the assembly line is different from the size and performance of the pipe product when it is used. The main reasons are: first, crystallization occurs during the cooling of the polyolefin melt. The crystallinity and crystal form are related to the temperature and thermal history, and the time of placement. Second, the temperature of the pipe just off the assembly line is usually higher than normal temperature. Third, the internal stress of the pipe just off the line is large. In order to achieve the performance and dimensional stability, the general polyethylene pipe should be placed off the line for 24 hours, and the polypropylene pipe should be placed for 48 hours, and the performance test can be performed according to the corresponding standard.
3. Common problems and treatment in the production of polyolefin pipes
Polyolefin melt is viscoelastic. Two phenomena often occur during processing, namely, die swell and melt rupture. Here, no detailed explanation. The following lists the common abnormal conditions in pipe production, their causes and treatment methods.
FAQ: The surface is dull and dull
cause:
1. Raw material moisture 2． The melt temperature is not suitable 3． 3. The molten material extruded by the extruder is not uniform. The sizing sleeve is too short 5． Die forming section is too short
Recommended solution
cause:
1. Raw material pretreatment 2． Adjust the temperature 3． 3. Increase the back pressure, use a finer filter, and design a suitable screw structure. Lengthened sizing sleeve 5． Lengthen the forming section of the die.
FAQ: Surface spots
1. There is moisture in the raw material 2． There are bubbles on the tube in the sink
Recommended solution
1. Dry raw materials 2． Eliminate bubbles. Adjust the process temperature.
Frequently Asked Questions: The outer surface appears bright and transparent block (commonly known as eye clear)
cause:
1. The temperature of the machine head is too high 2． The cooling water is too small or insufficient, or uneven 3． Lower the head temperature
4. Open the cooling water or clean the sizing sleeve
Frequently asked questions: regular markings on the smooth outer surface of the pipe
Cause: The pipe tends to adhere to the sizing sleeve, increase the flow of cooling water, clean the waterway or slow down
Cause: The deep corrugation of the outer surface of the pipe is not centered and centered, and the sizing box and the die are kept on the same axis
Common problems: rough inner surface
cause:
1. Raw material is moist 2． Low core mold temperature 3． The gap between the die and mandrel is too large. 4. The die shaping section is too short
Recommended solution:
1. Raw material drying, or pretreatment 2. Increase the temperature or extend the holding time 3. Change the core mold 4. Change the mold with a longer shaping section
FAQ: Corrugated inner wall
cause:
1. The output of the extruder changes and the feed is unstable. Traction slip 3． Uneven cooling of pipes
Recommended solution:
1. Reduce the temperature in the screw feed zone. 2. Adjust the traction air pressure. 3. Regulating waterway
FAQ: There are pits on the inner wall of the tube
cause:
1. The moisture of raw materials is large. Poor dispersion of the filler, unplasticized, impurities
Recommended solution:
1. Raw material preheating and drying 2． Refueling, adjusting temperature, cleaning raw materials
Frequently Asked Questions: There are coke particles on the inner wall of the tube
cause:
1. The extruder head and the inner wall of the die are not clean 2． Local temperature is too high 3． Serious die accumulation
Recommended solution:
1. Clean mold 2． Check if the thermocouple is normal. 3. Mold cleaning, lower the die temperature appropriately
Common problems: the outer diameter or wall thickness changes at any time
cause:
1. Extrusion speed change 2． The traction speed changes or slips 3． Unstable cutting (uneven particle size) 4． Melt instability 5． Uneven cooling
Recommended solution:
1. Check the tractor 2． Increase the pressure appropriately 3． 3. Sift or granulate the raw materials 4． 4. Increase the material temperature, reduce the line speed, and increase the die gap 5． Clear waterway
FAQ: Uneven wall thickness
cause:
1. The die is not centered 2． Uneven die temperature 3． 3. Tractor, sizing sleeve, die not centered 4． The distance between the sizing sleeve and the die is too far
Recommended solution:
1. Adjust the die to be concentric 2． Adjust the temperature 3． Keep on the same axis 4． shorten the distance
Common problems: bad welding seam
cause:
1. The die forming section is too short 2． Low melting temperature 3． 3. Disperse the plastic in the die head. The machine head structure is unreasonable
Recommended solution:
1. Use a longer die forming section 2． Raise material temperature 3． Clean the die head 4． Replace or rebuild
Frequently Asked Questions: The pipe is damaged prematurely
cause:
1. Blisters 2． Air bubbles 3. Impurities 4. Poorly dispersed pigments or fillers
Recommended solution:
1. Dry raw materials 2． Dehumidify or lower the temperature 3． 3. Clean the raw materials or use the filter 4． Adjust temperature or change raw materials
Frequently Asked Questions: Premature pipe damage and brittle failure
cause:
1. Low material temperature 2． The temperature is too high, the raw materials decompose
Recommended solution:
1. Raise the material temperature 2． Clean the mold and reduce the temperature
FAQ: Cracking of pipes
cause:
1. The head temperature is low and the extrusion speed is fast 2． The cooling water is too large 3． 3. Increase temperature and decrease speed 4． Reduce cooling water flow
Frequently Asked Questions: The pipe is not well rounded and bent
cause:
1. The center position of the die and core die is not correct 2． The temperature of the machine head is not uniform all around 3． 3. The cooling water is too close to the die 4． The cooling water spraying force is too large 5． The cooling water spray is too small 6． The water level is too high 7. Excessive pressure of tractor
Recommended solution:
1. Adjust concentricity 2． Adjust the temperature 3． Adjust the position of cooling water 4． Adjust the nozzle angle 5． Clear the waterway 6． Drainage 7. Adjust air pressure
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